1. Field of the Invention
The present invention relates to a data transfer apparatus which selects a specific channel from among a plurality of channels connected thereto and transfers data from a data processing apparatus to the selected channel, and a data transfer method.
2. Description of the Related Art
In most cases, a conventional data transfer apparatus which is connected to a plurality of channels to perform data transfer comprises an output buffer because the data reception speed of the channels is lower than the data output speed of a data processing apparatus, and output data to the respective channels is once held in the output buffer and thereafter transferred to a selected channel. The data transfer apparatus of this type cannot respond to a data output request to another channel, although the data processing apparatus itself is not in operation during a period of time between completion of transfer from the data processing apparatus to the output buffer and completion of transfer from the output buffer to the channel, with the result that the operation efficiency of the data processing apparatus is lowered.
As a conventional method of enhancing the operation efficiency of the data processing apparatus in such a data transfer apparatus, there has been such a method as disposing output buffers to the respective channels and starting data transfer to another channel as soon as transfer from the data processing apparatus to the output buffer is ended, thereby enhancing the operation efficiency of the data processing apparatus.
However, in such a method as disposing output buffers to the respective channels, it is necessary to prepare a considerably large-scale circuit in the case where there are a large number of channels. Moreover, a channel which does not need an output buffer by nature because the data reception speed thereof is high and a channel used with low frequency are also provided with output buffers, so that the circuit becomes less efficient for the scale thereof.
A method of enhancing the efficiency of data transfer to a multi-number of channels without such a large-scale increase of circuits is disclosed in Japanese Unexamined Patent Publication JP-A 5-28082 (1993).
The method disclosed in JP-A 5-28082 aims to reduce the scale of a circuit by classifying a plurality of data input channels into high-speed channels and low-speed channels and making an output buffer for the low-speed channels in common.
However, this method relates to a data input circuit and therefore cannot be applied as it is to a data transfer apparatus related to data output. Moreover, a buffer and a control circuit need to be designed in view of the properties of the respective channels, so-that the circuit becomes complex and less general-purpose.